A rechargeable battery is an important storage device of electric power, many devices, such as automobiles, motorcycles, agricultural machines and some spare power supply systems, depend upon such device. However a rechargeable battery is merely a power-storage device to supply a given amount of electric power, and is not a generator. In order to maintain its normal function, a rechargeable battery has to be connected with a power supply, such as a charging device or another battery for recharging.
Generally, it is not true that every user knows correctly about the positive and negative terminals of a rechargeable battery and about how to connect such a rechargeable with a recharging device. If a charging device is not well-designed or indicated correctly with a lamp or lamps, the user might damage the charging equipment and the battery, or even cause an accident.
For example, when the battery of a car is out of order, the car would be unable to start; then, another battery from another car has to be used to jump start the car. When the batteries of two cars are connected, the connection method looks simple, but danger might take place if the polarities of the two batteries are connected incorrectly. In that case, not only the battery will fail to supply the power desired, but the computer circuit and the battery might also be damaged. Moreover, serious accidents might result.
In a car repair shop, the repairman would use a special charging circuit to charge the battery of a car. For a novice repairman, the aforesaid danger might happen in case of negligence or failure to indicate the positive and negative terminals.